The present invention is generally directed to a control system, and, more specifically, to a control system for an electric heater.
Electric catalyst heaters (ECHs) have been proposed for use in a variety of applications, such as a micro-reformer, a main fuel reformer, electrically heated catalytic converters and in other applications that implement a low impedance heating element that requires temperature control such as furnaces and ovens. When implemented within a micro-reformer, an ECH is used to warm a reformer catalyst to initiate the reforming process. The fuel reforming process produces hydrogen, which can be used for fuel in a fuel cell or to help internal combustion engines (ICEs) meet super ultra low emission vehicle (SULEV) emission requirements. When implemented within a catalytic converter, an ECH typically also allows for reduced emissions due to light off (i.e. heating to a temperature where catalytic activity begins).
When implemented within a fuel reformer, a typical ECH element needs to be maintained at a temperature that corresponds to a very narrow resistance range. For example, an ECH may exhibit a resistance of thirty-five milliohms at 25xc2x0 C. and forty-one milliohms at 525xc2x0 C. As another example, a heating element may exhibit a resistance between twenty-five and one-hundred milliohms. Traditional resistance measurement techniques, which have involved using a bridge and a series measurement element, are generally ineffective at such low resistance levels. This is due to the fact that the series element must generally have a resistance much less than forty milliohms, a high power rating and allow for precise measurement with high accuracy. This combination of requirements is generally difficult and expensive to meet when attempting such resistance measurements with a bridge that uses a series measurement element.
What is needed is a control system that performs temperature control of a low impedance series element that is practical, economical and relatively accurate.
An embodiment of the present invention is directed to a control system for an electric heater, e.g., an electric catalyst heater (ECH), that includes a current sensor, a voltage sensor, a switching element, and a control circuit. The current sensor is positioned to sense a heating current through a heating element and the voltage sensor is positioned across the heating element to sense a heating voltage. The switching element includes a control terminal and is coupled between the heating element and a power supply to interrupt the heating current, through the heating element, responsive to a control signal on the control terminal. The control circuit is coupled to the current sensor, the voltage sensor and the control terminal of the switching element and varies the control signal to maintain the temperature of the heating element within a predetermined temperature range.
According to one embodiment of the present invention, the current sensor is a non-contact inductive current sensor. According to another embodiment of the present invention, the control circuit includes a microcontroller. According to the present invention, a temperature sensor is not required to monitor the temperature of the heating element and the heating element can be monitored without adding a costly high power series element, which, when implemented, produces additional heat. Further, when a microcontroller is utilized, high accuracy can be achieved through high-speed measurements. In one embodiment, the microcontroller is programmed to prevent overheating and subsequent damage of the heating element. A control system, according to the present invention, can be utilized with a heating element that requires rapid warm-up for use in various applications such as micro-reformers for solid oxide fuel cells (SOFCs).
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.